Swivel

ABSTRACT

A swivel assembly for installation at a well for subsea production of oil or gas and adapted to be connected to a production vessel at the sea surface. The swivel assembly comprises a main fluid swivel having at least two paths and an electric/hydraulic auxiliary swivel for signal communication and power transfer. The fluid swivel is provided with a rotatable swivel housing at the top of a stationary christmas tree, preferably comprising a small number of valves, such as a production master valve and an annulus master valve. The paths are through-running vertically in the central core member of the fluid swivel so as to make possible well intervention from the upper side of the fluid swivel.

FIELD OF THE INVENTION

This invention relates to a swivel device for installation at a well forsubsea production of oil or gas and adapted to be connected to aproduction vessel at the sea surface, comprising a main or fluid swivelhaving at least two fluid passages and an electric/hydraulic auxiliaryswivel for signal communication and power transfer.

BACKGROUND OF THE INVENTION

When developing marginal offshore production fields it is important tokeep the costs low. A substantial cost factor with known forms ofdevelopment, is due to subsea pipelines and cables between christmastrees at the wells and product receiver sites, for example a platform ora floating production vessel. Typically there may here be the questionof distances like 2 km. In this connection it is to be noted that moderndrilling technology makes it possible to produce by means of a reducednumber of christmas trees, because the boreholes or wells can haveseveral branches. This will make it possible to develop subsea fields insome instances by employing only one subsea christmas tree.

International patent application PC/NO96/00201 dated Jul. 8, 1996relates to a system for offshore production of hydrocarbons by employinga moored production vessel or ship. The present invention can beregarded as a further development thereof and is based on the idea thatthe production vessel can by lying directly above or in the immediatevicinity of the christmas tree at the seabed and that the well fluidflows are transferred directly between the christmas tree and the vesselwithout relying on pipelines or cables on the seabed.

Advantageously the solution to be described in the followingspecification can employ a particular and preferably simplified form ofchristmas tree, being more or less integrated into an arrangementaccording to the invention as will appear from the followingdescription.

Thus, in a device as stated in the introduction above, the novel andspecific features according to the invention primarily comprise thereinthat the fluid swivel is provided with a rotatable swivel housing at thetop of a stationary christmas tree, preferably comprising a small numberof valves, such as a production master valve and an annulus mastervalve, and that said passages are through-running vertically in acentral core member of the fluid swivel in order to make possible wellintervention from the upper side of the fluid swivel.

On the basis of this fundamental solution, which in actual practice canbe embodied in various forms, it is possible to obtain a number ofadvantages, of which in particular the following are mentioned:

The production vessel employed does not need any modifications worthmentioning, in relation to common ship designs, and therefore will berelatively inexpensive,

the same production vessel can be used for installing and possiblyretrieving the swivel device, or also for well maintenance, whichcontributes to reduced operational costs,

very small marginal subsea fields can be profitable by using this novelarrangement, so that the degree of extraction can be increased for thefields,

the equipment employed can be re-used by moving it from field to field.

SUMMARY OF THE INVENTION

The present invention is a swivel device for installation at a well forsubsea production of oil or gas and adapted to be connected to aproduction vessel at the sea surface comprising a main fluid swivelhaving at least two passages and an auxiliary swivel. The swivel deviceis provided with a rotatable swivel housing on top of a stationarychristmas tree and that the passages are through-running vertically in acentral core member of the fluid swivel, in order to make possible wellintervention from the upper side of the fluid swivel. Further, at thelower part of the christmas tree, there is provided a force transferringhousing being rotatably journalled about a common vertical axis on abase structure. The swivel device further comprises a mooring yokehaving two legs, the free ends of which are rotatably connected toeither side of the force transferring housing, and the opposite ends ofwhich are adapted to be attached to the end of a mooring line. Betweenthe force transferring housing and the swivel housing, there is provideda mechanical coupling for causing the swivel housing to perform the samerotary movements as the force transferring housing under the influenceof the yoke.

Further, the mechanical coupling between the force transferring housingand the swivel housing comprises a rod element extending in parallel tothe common vertical axis at the side of the christmas tree and theswivel housing.

The swivel device of the present invention may further comprise aretrievable top member incorporating the auxiliary swivel. The topmember at its lower part is provided with a connector adapted tocooperate with the core member of the fluid swivel.

The top member may further contain a control module for the christmastree and may be provided with a handling head for installation andretrieval.

The swivel device of the present invention may further comprise abridging arm extended outwards and downwards from the top member andprovided with a connector plate at its lower end. The connector plate isadapted to cooperate with a complementary connector part associated witha laterally projecting connecting member on the swivel housing.

A further embodiment of the instant invention provides the swivelhousing with laterally directed connecting members for at least tworisers being oriented substantially in the same plane as a connectingmember for an umbilical. The connecting members are directedsubstantially outwards centrally between the yoke legs.

The swivel device may be characterized such that the wellhead comprisesa conductor tube, characterized in that the force transferring housingis rotatably provided on a base structure being connected to theconductor tube for transferring mooring forces directly from the yoke tothe conductor tube.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following description the invention will be explained moreclosely with reference to the drawings, in which:

FIG. 1 shows a simplified overview of a total arrangement with aproduction vessel associated with an installation at the seabed andbeing moored thereto, and

FIG. 2 shows an example of a preferred embodiment of the deviceaccording to the invention, as seen in elevation and partially insection.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows an overview of the total arrangement when a productionvessel or ship 3 operates in association with a well or a similarinstallation as generally shown at 20 on the seabed 1. Risers or hoses31 and 32 are shown here in their full length up to the ship 3, wherethey are connected to a processing unit 3A being shown purelyschematically. Moreover a mooring line 69 is extending up through thesea surface 2 to mooring means at the bow portion of the ship 3. Anintermediate region of the mooring line 69 is provided with a buoyancyelement and likewise the risers 31 and 32 have buoyancy bodies at alower portion for elevating the risers or hoses from the bottom 1. Thisgeneral system is described more closely in the above mentionedInternational patent application.

In FIG. 2 there is indicated with hatching those parts and componentswhich take part in the swing or turning movement when a productionvessel moves around a subsea installation under the influence of windand weather, in particular waves. The arrangement is preferably built upwith a central and common vertical axis 20X coinciding with the axis ofa wellhead 48, i.e. the top of the production tubing, when a subsea oilor gas well is concerned.

FIG. 2 in relatively much detail primarily shows more or less commonstructures at the top of an oil or gas well at the seabed 1. A concreteplate or base 45 forms a foundation on seabed 1 and has a centralopening for supporting a self-adjusting ball joint type frame 46 whichin turn is connected to a common conductor tube 47. In a known mannerthis can extend for several tens of meters down into the structureunderneath the seabed 1. Within conductor tube 47 the actual productiontubing is hanging, and ending at the top in the wellhead 48 itself.

A base structure 49 serves for supporting the whole arrangement aboveit. Through strong, supporting structure elements the base structure 49is directly connected to the conductor tube 47 for transferring mooringforces thereto. Journalled at the base structure 49 there is shown astrong, carousel-like housing 60 being rotatable about the central axis20X. Housing 60 is provided with preferably diametrically opposedattachment members 61 in the form of projecting studs for a yoke 63, theupper end 64 of which is adapted to be connected to one or more mooringlines 69. It will be seen that yoke 63 has two legs being united at anupper end or cross piece 64. Yoke 63 can assume various angularpositions by swinging about the horizontal axis extending diametricallybetween the attachment members 61, whereby the angular range of the yokemovement at least extends upwards to an approximate vertical position,whereas the lowermost angular position in practice is restricted in viewof umbilical 33 and/or risers 31 and 32.

During operation with a moored production vessel, this, through theintermediary of line or lines 69 and thereby yoke 63, will causerotation of the hatched parts of the arrangement about the axis 20X. Acorresponding rotary movement takes place in the actual swivel device,whereby the central core member 25 is stationary together with valveblock 50, whereas the remaining parts of the swivel including the swivelhousing 24 participate in the rotary movement. Thus, swivel housing 24is journalled at 30 on top of valve block 50.

In order to secure a simultaneous and common rotary movement of housing60 and swivel housing 24, there is shown a mechanical guide or couplingin the form of an upright rod element 72 being supported at its lowerend by a bracket 71 attached to housing 60, and further up extendingthrough a sleeve 73A on a supporting arm 73 projecting laterally fromswivel housing 24. This mechanical coupling has been made sufficientlyrigid for swivel housing 24 to perform the same rotary movements as thecarousel or housing 60 under the influence of yoke 63, when this isconnected to a surface vessel through mooring line 69. In addition tothe rotary interconnection between housing 60 and the fluid swivelhousing 24 as just explained, rod element 72 in cooperation with sleeve73A can also have a useful and advantageous function during installationof the swivel device on a wellhead. For this purpose sleeve 73A has adownward wide cone 73B for cooperation with the upper end of rod element72 when landing the fluid swivel and possible christmas tree 50 onwellhead 48. The christmas tree or block 50 at its lower part has amounting or connector device 50B for detachable mounting on wellhead 48,as previously known per se.

Swivel device 50 has a stationary, central core member 25 with axiallythrough bores or passages comprising a production bore 21 and an annulusbore 22. Both of these communicate downwards with corresponding fluidpassages or conduits in valve block 50, for the purpose, inter alia, ofthe various valve functions made possible thereby. Specifically it is tobe noted in this connection that these fluid bores or passages extendvertically from the top of the central core member 25 of the swivel andpreferably with direct prolongation in corresponding passages 56,57 asindicated in christmas tree 50. Thus, well intervention from the upperside of the fluid swivel is made possible. This however, is on conditionthat a top member 77 of the total swivel device is removed, as will bedescribed more closely below.

Moreover, around core member 25 the swivel device 20 has two or moreannular fluid passages with associated seals and bearing elements, asgenerally shown at 27. These parts of a fluid swivel are previouslyknown per se, for example from Norwegian patent No. 177.780,illustrating an axially separable swivel device, being primarilyintended for other uses.

Swivel housing 24 is provided with connecting members generally denoted31A and 32A for risers or hoses 31 and 32 adapted to extend upwardsthrough the water to the production vessel. (See FIG. 1). Whereasconnecting members 31A and 32A for fluid transfer are located relativelycentrally at swivel 20, an upper connecting member 33A for an umbilical33 is located at an upper part of swivel housing 24, being adapted torotate about axis 20X during the turning movements mentioned. A bearing30 supports swivel housing 24 so that it can rotate in relation to thechristmas tree or block 50.

By means of a connector 20B a top member represented by its encasing 77is incorporated in the total swivel device 20 in FIG. 2. Connector 20Bpermits separation of top member from and retrieval thereof from theremaining swivel device, as will appear from the following description.An auxiliary swivel 28 for transferring signals and power, is located intop member 77 and this also accomodates a control module 29 for thechristmas tree. A slip ring mechanism 79 serves for the required rotarycoupling of the various wire connections between said components in topmember 77 and christmas tree 50, possibly also other units beingincorporating in the wellhead.

On the other hand there are provided electric and hydraulic connectionsfrom top member 77 through a bridging arm 77B extending from a sidewallof the top member encasing 77 outwards and downwards along the side ofconnector 20B, and provided with a connector plate 78 or the like forcooperation with a complementary connector part on a connecting member33A for umbilical 33. Accordingly, this can, through connection 78,bridge arm 77B and the interior of top member 77 including slip ringmechanism 79, communicate with christmas tree 50 and so forth.

As indicated with reference numeral 26 at the lower part of top member77, this is rotatably journalled on connector 20B. A synchronous andsimultaneous rotary movement of top member 77 under the influence ofyoke 63 is secured by means of rod element 72, entering into a sleeve75A connected to the top member encasing 77 through a radial arm 75.Sleeve 75A has a downwardly open cone 75B which like cone member 73Bbelow, contributes to a safe landing and installation of top member 77separately on the upper end of core member 25, at the same time asconnection is established at 78. For such installation or retrieval oftop member 77 this is provided with an upper handling head 77A, which ina manner known per se can be configured and dimensioned for cooperationwith suitable tools. It is obvious that these operations by means ofhandling head 77A can also depending on the circumstances, comprise thewhole swivel device, including the fluid swivel 24,27 and the christmastree or block 50 as well as connector 50B thereon, for installation atwellhead 48.

The laterally directed connecting members 31A and 32A for risers arepreferably oriented in the same plane as connecting member 33A forumbilical 33, and all connecting members are preferably directedsubstantially outwards from swivel housing 24 centrally between the yokelegs 63. It can be practical to provide isolation valves 31B and 32Bbetween the connecting members and swivel housing 24, as will appearfrom FIG. 2. These isolation valves can also act as wing valves, whichcan be of interest in view of the preferred, simple form of christmastree 50.

What is claimed is:
 1. A swivel device for installation at a well forsubsea production of oil or gas and adapted to be connected to aproduction vessel at the sea surface, comprising: main fluid swivelhaving at least two passages, a central core member, and an auxiliaryswivel, wherein the fluid swivel is provided with a rotatable swivelhousing on top of a stationary christmas tree; wherein said passages arethrough-running vertically in said central core member of the fluidswivel, in order to make possible well intervention from the upper sideof the fluid swivel; a force transferring housing at the lower part ofthe christmas tree, said force transferring housing being rotatablyjournalled about a common vertical axis on a base structure; a mooringyoke having two legs, the free ends of said legs are rotatably connectedto at either side of said force transferring housing, and the oppositeends of said legs adapted to be attached to the end of a mooring line;and a mechanical coupling provided between said force transferringhousing and said swivel housing for causing said swivel housing toperform the same rotary movements as said force transferring housingunder the influence of said yoke.
 2. The swivel device according toclaim 1, wherein said passages in the core member are aligned in directprolongation of corresponding passages in the christmas tree.
 3. Theswivel device according to claim 2, wherein said core member projectssomewhat upwardly from the upper side of the swivel housing.
 4. Theswivel device according claim 2, further comprising a retrievable topmember incorporating said auxiliary swivel, said top member being at itslower part provided with a connector adapted to cooperate with the coremember of the fluid swivel.
 5. The swivel device according to claim 2,wherein said swivel housing is provided with laterally directedconnecting members for at least two risers being oriented substantiallyin the same plane as a connecting member for an umbilical, saidconnecting members being directed substantially outwards centrallybetween the yoke legs.
 6. The swivel device according to claim 2,whereby the well includes a wellhead which comprises a conductor tube,wherein said force transferring housing is rotatably provided on a basestructure being connected to said conductor tube for transferringmooring forces directly from said yoke to the conductor tube.
 7. Theswivel device according to claim 1, wherein said mechanical couplingbetween said force transferring housing and said swivel housingcomprises a rod element extending in parallel to said common verticalaxis at the side of the christmas tree and the swivel housing.
 8. Theswivel device according to claim 7, wherein said rod element is attachedat its lower end to said force transferring housing and cooperates witha sleeve at the outer end of an arm the inner end of which is attachedto said swivel housing, said sleeve having a downward guide cone forcooperation with the upper end of said rod element.
 9. The swivel deviceaccording to claim 7, whereby the well includes a wellhead whichcomprises a conductor tube, wherein said force transferring housing isrotatably provided on a base structure being connected to said conductortube for transferring mooring forces directly from said yoke to theconductor tube.
 10. The swivel device according to claim 1, furthercomprising a retrievable top member incorporating said auxiliary swivel,said top member being at its lower part provided with a connectoradapted to cooperate with the core member of the fluid swivel.
 11. Theswivel device according to claim 10, wherein said top member isrotatably journalled on said connector.
 12. The swivel device accordingto claim 11, wherein said top member contains a control module for saidchristmas tree.
 13. The swivel device according to claim 11, wherein abridging arm is extended outwards and downwards from said top member andis provided with a connector plate at its lower end, adapted tocooperate with a complementary connector part associated with alaterally projecting connecting member on said swivel housing.
 14. Theswivel device according to claim 11, wherein said top member is providedwith a handling head for installation or retrieval of said top member.15. The swivel device according to claim 10, wherein said top member isprovided with a handling head for installation or retrieval of said topmember.
 16. The swivel device according to claim 10, wherein said topmember is provided with an arm carrying a sleeve with a cone adapted tocooperate with said rod element.
 17. The swivel device according toclaim 1, wherein said swivel housing is provided with laterally directedconnecting members for at least two risers being oriented substantiallyin the same plane as a connecting member for an umbilical, saidconnecting members being directed substantially outwards centrallybetween the yoke legs.
 18. The swivel device according to claim 17,wherein between said connecting members for risers and the swivelhousing, there are provided isolation valves.
 19. The swivel deviceaccording to claim 1, whereby the well includes a wellhead whichcomprises a conductor tube, wherein said force transferring housing isrotatably provided on a base structure being connected to said conductortube for transferring mooring forces directly from said yoke, to theconductor tube.
 20. The swivel device according to claim 1, wherein saidcore member projects somewhat upwardly from the upper side of the swivelhousing.